Percentile alpha control in the Compositor?
Yes by using math.

The alpha channel is a value from 0.0=transparent to 1.0=solid. A math node can be used to multiply alpha values by the value you want to use. Which you then feed into the alpha over Fac to combine the images. You can also use a CombineRGBA to join together depending on your needs.

While multiplying a zero value by 0.5 leaves it zero you can use math node to get values greater than 0.99 if you wish to only effect a non-transparent area.

You can also use less than to pass through unaltered values and add them together before the alpha over.